US2436834A - Phase and frequency modulation - Google Patents

Description

March 2, 1948.

Filed Nov. 17, 1942 3 Sheets-Sheet 1 A c o F G L|M|TER LIMITER SOURCE OF GOMBINER couamzn g To ANY FIXEDPHASE QO'PHASE OF CARRIER 90' PHhSE OF CARRIER AMTONAL AND SHIFTER flSIDE BAND SHIFTER a BAND Mg??? FREQUENCY VECTOR a vsc on a TRANSWTTEF';

BUFFER BUFFER '''m""fi%%%" E "sane aAms'ARE moss WHICH RESULT FROM PASSING THE PHASE MODULATED WAVE BALANCED BALANCED mg MODULATOR MODULATOR FIGJ IN VENTOR ATTORNEY E. K. STODOLA' PHAS E AND FREQUENCY MODULAT I ON Filed Nov. 17, 1942 5 Sheets-Sheet 2 INVENTOR EDWIN K. STODOLA W M A TTORNE Y March 2, 1948.

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PHASE AND FREQUENCY MODULATION Filed Nov. 17, 1942 3 Sheets-Sheet 3 FIG. 5

IN VEN TOR E EDWIN KQS'TODOLA Y Wad- Q.

ATTORNEY Patented Mar. '2, 1948 PHASE AND FREQUENCY MODULATION Edwin E. Stodola, Neptune, N. .1. Application November 17, 1942, Serial No. 465,920

6 Claims.

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) This invention relates to modulation systems, particularly those of the phase and frequency modulation type.

Most phase modulation systems permit change of phase which is linear with respect to modulation voltage over a relatively small range of phase displacement. To obtain larger phase shifts, it has been the practice to pass the phase shifted wave through a series of multipliers and heterodyne stages to get a desired phase shift at a desired value of mean carrier frequency. This results in a large amount of complication and a loss in frequency stability of the final carrier wave.

It is an object of this invention to devise a phase or frequency modulation network in which linear phase displacement over larger ranges is made possible, thus permitting a reduction in the number of frequency multipliers and the elimination of some or all of the heterodyning stages.

More specifically, it is an object of this invention to provide a cascaded modulation system in which the first stage provides a portion of the phase shift and the resultant phase shifted wave is further shifted in phase by the succeeding stages.

It is a further object of the invention to provide a novel cascaded phase or frequency modulation system wherein undesirable amplitude modulation is eliminated or neutralized.

Further objects will be apparent from the specification and drawings, wherein Figure 1 is a simplified diagram of one form of I modulation system.

Referring to Figure 1, the output of a relatively fixed frequency carrier voltage source A is applied to a balanced amplitude modulator B upon which the modulating wave is also im= pressed. The modulator B is of the type in which part, and preferably all, of the carrier is suppressed and the output of the modulator contains modulation side bands the amplitude of which vary in proportion to the modulating voltage amplitude. Modulator B may be any conventional type of carrier suppressing modulator or it may be of the type described and claimed in my copending application entitled Modulation networks, Serial No. 465,922, filed on even date herewith.

The output of carrier source A is also passed thru a fixed phase shifter C and then combined with the output of modulator B in combining network D. The resultant output will be a phase modulated wave the mean frequency of which is the same as that of source A. Network D may also contain conventional buffer amplifiers and amplitude limiters to remove any undesirabie amplitude modulation that may be present.

As thus far described, the system is conventional. With such a system it is, however, possible to obtain phase shifts which are substantially linear with respect to modulation voltage through a range of only about thirty degrees. To provide large phase shifts it has been-the custom to multiply the frequency of the phase shifted wave several thousand times. Since this process also multiplies the mean frequency of the carrier it has been the practice to interpose in the muitiplying stages one or more hetercdyning stages wherein the output of a fixed oscillator is heterodyned with the multiplied frequency and the difference frequency is again multiplied and/or amplified and finally radiated.

However, this process involves use of two or more oscillators and the frequency stability of the final carrier is the algebraic sum of the stabilities of all the oscillators. Consequently, the number of heterodyning stages used and therefore the maximum amount of phase shift that can be obtained with this method is limited, if a very stable carrier frequency is required, unless resort is had to complicated and cumbersome techniques.

In the present invention, the substantially linear phase shift can be almost doubled by sub- J'ecting the phase modulated ouput of network I) to a further modulation in networks E, F, and G which function in the same manner as networks B, C, and D respectively. The same modulating wave controls both modulators B and E. Any number of phase modulators can be connected cascade as above described. Thus large linear phase shifts can be'obtained from one carrier source, permitting the reduction in the number of frequency multiplication stages and reduction or elimination of the number of heterodyning stages.

Figure 2 illustrates another embodiment of my invention in which use is made of a type of phase modulator such as is described and claimed in my copending application, entitled Modulation system, Serial No. 652,210, filed March 5, 1945, which is in turn a division of my application entitled Electrical networks, Serial No. 465,921, filed November 7, 1942, which has matured into Patent No. 2,397,992, issued April 9, 1946. For an eX plantion of this modulator, reference is made to Figure 3.

If a fixed voltage V is impressed upon a reactor 29 in series with a resistor 3, the output voltage V will be phase shifted with respect to V by an angle whose tangent is equal to the ratio of the reactance divided by the resistance. If such reactance is capacitative the output voltage will lead the input voltage. If the reactance is inductive, it will lag the input voltage.

If resistor 3 is the anode-cathode space of a triode, or other form of electron tube, to the grid of which is applied a modulating voltage, and input voltage V is derived. from a carrier source, then the output voltage V0 will be phase modulated. However, the phase of the output voltage will not, in general, be a linear function of the modulating voltage, but the function can be made substantially linear over the operating range by choosing the proper value of series reactance for a given tube resistance.

Figure 4 shows a family of curves 3iia3td, representing the change in phase angle between the input voltage V and the output voltage V0 with respect to changes in grid bias of a type 605 tube having different values of reactance in series with the anode-cathode path of the tube. In the curves shown, the tube is operated with a plate polarizing potential of 250 volts D. C.

Curve 30b shows that with a reactance of 15,009 ohms in series with the tube a linear relationship exists between grid voltage and phase change for a range of 15 to 55.

Referring to Figure 2, the output of a relatively fixed frequency oscillator A is applied to a phase modulating network J, similar to that shown in Figure 3, in which the reactance 29 is a condenser 29a and the variable resistor 3 is the anode-cathode path of a triode 3a connected in series with condenser 29a. A source of D. C. plate potential I5 is connected to the plate of the triode through an R. F. choke [6. To neutralize or minimize the effect of the anode-cathode capacity of the triode, a condenser l8 and inductance I1 is connected across the anode and cathode path to form with said path a parallel resonant circuit which is resonant at, and presents a very high impedance to, the frequency of the current impressed thereon from oscillator A. A blocking condenser l5, of negligible impedance to the impressed frequency is interposed in the anode circuit to prevent shorting of the plate battery through coil l'l.

The output of modulator J is impressed upon a second modulator M directly or through one or more buffer amplifiers K. Condenser I9 is a blocking condenser of negligible impedance to the impressed frequency.

Modulator M is similar to modulator J, except that reactance 29b is inductive. The phase shifting characteristics of the two modulators are therefore opposite in sense.

To make the total phase modulation cumulative, the grids of the two modulators are driven in opposite phase by a differential modulatin circuit including the push-pull secondary oi transformer 2t, on the primary of which is im pressed the modulating voltage; A steady negative bias is impressed upon the grids of both modulator grids by battery 2i connected between ground and the center-tap of the transformer secondary.

The output of modulator M can also be impressed upon further cascade-connected phase modulators, such as above-described, and then frequency multiplied and radiated. Or it may first be frequency multiplied and then passed through more phase modulator stages operating at the multiplied frequency. Amplitude limiting can also be introduced at any stage in the cascade if undesirable amplitude modulation is present.

The circuit in Figure 2 permits of another ad vantage. Most phase or frequency modulators also amplitude modulate the impressed wave. This undesirable by-product is usually eliminated by one or more amplitude limiting stages. Reference to curve see in Figure 4 will indicate the amount of amplitude variation when reactance 29 has a value of 15,000 ohms. By modulating each pair of modulators in opposite phase, as is done in Figure 2, the amplitude modulation is largely cancelled. This method of reducing the amplitude modulation can also be applied to a single stage phase modulator. This can be done by impressing the modulation voltage in reversed phase upon an amplitude modulator upon which the phase modulated wave is impressed, e. g. upon the control grid of the tube in circuit K of Figure 2.

The curves in Figure 5 show the characteristics of the system above-described compared with those of a single stage modulator. Curves 5a and 50 show the range of amplitude and linear phase variation, respectively, of a single unit modulator. Curves 5b and 512, respectively, show the smaller amount of undesirable amplitude modulation and the larger amount of linear phase modulation of the novel system herein described.

It should be noted, in general, that phase modulators are not linear over their whole operating range. Where several like modulators are cascaded, their non-linearities and amplitude modulators are cumulative. It is therefore desirable, where a large number of modulators are cascaded, to use modulators having undesirable characteristics which tend to neutralize each other. In Figure 2 this has been applied par ticularly with respect to amplitude modulation.

Although reference has been made to phase modulation, it is appropriate to refer to the fact that pure frequency modulation is equivalent to phase modulation in which phase shift for a given modulating voltage is inversely proportional to the modulating frequency. At any single modulating frequency there is no distinction between the two. Consequently, where reference has been made to phase moduation, it should be understood that by appropriate control of the modulating voltage the same methods and devices can be used to produce frequency modulation. In the claims, the term wave-length modulation Will be used as generic to both types of modulation.

It should be understood that the specific circuits above described are merely illustrative of the principles of the invention. Many modifications may be made without departing from amazes the spirit of the invention as defined in the appended claims.

Iclaim:

1. A wave-length modulation system for a source of oscillations of relatively fixed frequency comprising a pair of phase modulators, one of said modulators comprising a capacitor and resistor connected in series to said source, a coupling circuit connected across said resistor and to the other modulator, said other modulator comprising an inductor and resistor connected in series to said coupling circuit, and means for varying the resistances of said resistors in opposite senses.

2. A system as set forth in claim 1, wherein the reactances of said capacitor and. inductor, at the frequency of said source, are equal and the resistances of said resistors are identical,

3. A system as set forth in claim 1, wherein each resistor is constituted by the anode-cathode path of an electron tube.

4. A system as set forth in claim 1, wherein each resistor is constituted by the anode-cathode path of an electron tube and means for reducing the effect of the anode-cathode capacity.

5. A wave-length modulation system for a source of oscillations comprising a first pair of series-connected impedances across said source, a second pair of series-connected impedances coupled across one of the impedances of said first. 30

pair, an output circuit coupled across one of the impedances of said second pair, one impedance in each pair being resistive, the remaining impedance in one pair being inductive, the remaining impedance in the other pair being capacita- Ltive, and means to differentially vary said resistive impedances.

6. A system as set forth in claim 5, wherein said inductive and capacitative impedances are of equal reactance at the frequency of said source,

REFERENCES CITED The following references are of record in the .file of this patent:

UNITED STATES PATENTS Number Name Date 2,045,107 Shore June 23, 1936 2,075,071 Usselman Mar. 30, 1947 2,104,318 Crosby Jan. 4, 1938 2,143,386 Roberts Jan. 10, 1939 2,160,528 Usselman May 30, 1939 2,173,145 Wirkler Sept. 19, 1939 2,231,079 Longo Feb. 11, 1941 ---2,254,734 Falloon et a1, Sept. 2, 1941 2,281,935 Hansen May 5, 1942 2,294,372 Barton Sept. 1, 1942 2,321,269 Artzt June 8, 1943 Certificate of Correction Patent No. 2,436,834. March 2, 1948. EDWIN K. STODOLA It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Column 4, lines 49 and 50, for the word modulators read modulations; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 1st day of June, A. D. 1948.

THOMAS F. MURPHY,

Assistant Oommtisszoner of Patents.

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